I've been working on a 4-stat system, too. The four stats that I picked were:
Agility
Brutality
Cunning
Insight
They get combined to create derivative stats:
Agility + Brutality = Stamina, which is spent to power Rote Magic (i.e. Bo9S maneuvers), and min(A,B) = per-turn Stamina regen. It's also used as your Fortitude defense.
Agility + Cunning = Wit, which is used as both a social defense and a reflex defense, but it can be lowered by repeated attacks -- it's effectively an ablative defense that regenerates quickly out of combat, like how Vitality works in Vitality/Wound systems, or how Stress works in FATE. Just like Wit, it regenerates per-turn at min(A,C).
Agility + Insight = Defense, which is your basic damage avoidance stat.
Brutality + Cunning = Crit, which is a rough measure of how awesome your critical hits are. This applies to magic, too.
Brutality + Insight = Accuracy, which applies to weapons and magic alike.
Cunning + Insight = Resolve, which is your Willpower. Just like Wit and Stamina, it's also an ablative defense (of the Will type). Similar to Wit and Stamina, your Resolve regenerates per-turn at min(C,I).
There's no magic stat as such, but rather all stats are applicable to magic -- some schools favor one stat over the others, like Evocation is good at damage, and rewards an investment in Brutality -- but it's always possible to find a good use for a high stat.